The present invention relates to a light scanning apparatus (light scanner), and more particularly to a light scanner suitable to a color laser beam printer or multi-color laser beam printer having an electronic photographing process, which uses a plurality of laser beams to scan a plane which carries images.
In a prior art light scanner of a color laser beam printer (color LBP), a plurality of laser beams is used to scan an image carrier plane to write an image.
In general, in such a light scanner, a single laser beam is directed to each deflecting reflection plane of a light deflector which comprises a single polygon mirror, and an f-.theta. lens is provided for each of the laser beams reflected by the deflecting reflection plane. The light beam from the f-.theta. lens is used to scan the image carrier plane by utilizing an anamorphic plane having corrected for a skew of the deflecting reflection plane. Since one set of scanning optical system is provided for each of the laser beams, the entire apparatus tends to be of large size and complex.
In Japanese Laid-Open Patent Applications 61-92917 and 58-79215, two lights having different polarization characteristics are used or two lights having different wavelengths are utilized, and the two laser beams are combined into one beam. The laser beam is then focused by one half as many lens systems as the number of laser beams and the laser beam is directed to a vicinity of the plane to be scanned. Then, the laser beam is split into two by a polarization beam splitter or dichroic mirror to scan the image carrier planes.
In this method, since the two laser beams are combined and then it is split, the entire system is complex. Further, if a light scan angle on the scan plane is large, a light leakage takes place due to an incident angle characteristic of the polarization beam splitter or dichroic mirror when the lights are combined or split. As a result, the light scan angle should not be too large.
In U.S. Pat. No. 4,561,717, a plurality of laser beams are obliquely directed to a single deflecting reflection plane 520a of a single light deflector 520 having a rotating polygon mirror, applying a field angle at a right angle relative to light scan directions of the scan planes 561 and 562, as shown in FIG. 1. It is focused by an f-.theta. lens 530 which comprises a spherical lens system and split into a plurality of light beams by optical systems such as mirror systems 541 and 542 arranged at spaced positions from the laser beam, and then they are directed to image carrier planes 561 and 562 for scanning.
The laser beam obliquely directed to the f-.theta. lens 530 which comprises the spherical lens system causes bending of the scan line on the image carrier plane due to the optical characteristic of the f-.theta. lens. In the prior art, cylindrical lenses 551 and 552 are arranged in front of the image carrier plane to compensate for the bending of the image plane.
In this method, since the light beam is directed to the cylindrical lens with a scan angle (an image angle in the scan direction), an apparent refractive power of the cylindrical lens increases as the scan angle increases and the laser beam is focused in front of the scan plane. Namely, the bending of the image plane increases and spot diameters of the laser beam at the center of the scan range and a peripheral point are different from each other.
Where three or more laser beams are used, there are two or more oblique incident angles to the f-.theta. lens which comprises the spherical lens. If the oblique incident angles are different from each other, the f-.theta. lens characteristics of the scan line differ from each other due to the f-.theta. lens characteristics.
Namely, as shown in FIG. 1, since the skewed light beam is directed to the f-.theta. lens 530 which comprises the spherical lens system, the f-.theta. characteristic is distorted and changed depending on the oblique incident angle.
The f-.theta. characteristic of the scan line in the scan direction relative to the oblique incident angle does not linearly change. For example, ##EQU1## where X(.phi.=.phi..sub.0)(.theta.) is a coordinate of the scanning light beam in the scan direction for a scan angle .theta. of a light deflector when an oblique incident angle is .phi..sub.0, f is a focal distance of the f-.theta. lens and .alpha. is an incident angle of the skewed light beam.
As a result, it is not possible to align the scan lines directed to the f-.theta. lens at different incident angles. Because of this disadvantage, where scan lines corresponding to different color developments are to be superimposed in a color LBP where a high precision of multi-color registration is required, color distortion takes place. For example, as shown in FIGS. 2 and 3, if scan lines having oblique incident angles of 2.5 degrees and 7.5 degrees are to be superimposed by correcting the bendings of the scan lines by the same second collimator lens (cylindrical lens), there occurs a displacement of 0.6 mm in the length of the scan line at the scan angle of 30 degrees (scan position 160 mm) with the f-.theta. lens which comprises the spherical lens system having a focal length of 313.55 mm, as shown in FIG. 4. If a correction is made to align the scan points in the vicinity of the scan angle of 30 degrees by the magnification in the scan direction, there occurs a displacement of approximately 60 .mu.m in the scan line in the vicinity of the scan angle of 16 degrees, as shown by a curve a in FIG. 3. Even if balance is adjusted in the .+-. directions, there occurs a displacement of .+-.30-40 .mu.m as shown by a curve b in FIG. 3. This corresponds to a displacement of one half pixel in a printer having a resolution of 400 DPI.
It is an object of the present invention to provide a light scanner which directs a plurality of laser beams to one scanning deflector to scan a plurality of scan planes with a good f-.theta. characteristic over an entire scan range, with a good optical characteristic with a small bending of image plane, and with a small displacement for the superposition of the scan points.
In accordance with the present invention, a plurality of laser beams are deflected by the single light deflector and directed to a first focusing system and focused by a first focusing lens. They are then directed to second focusing lenses which comprise anamorphic systems, arranged one for each of the laser beams in a vicinity of the scan plane. The laser beams from the second focusing lenses are directed to the respective scan planes. Refractive powers of the first focusing lens in the scan direction and the vertical direction on the scan plane are afocal.